1. Field
One or more exemplary embodiments of the present invention relate to an optical modulator and methods of manufacturing and operating the same, and more particularly, to an optical modulator having a gating function, methods of manufacturing and operating the same, and an optical apparatus including the optical modulator.
2. Description of the Related Art
An additional unit is used in order to obtain information about the distance between a camera that takes two-dimensional images and a subject. The information about the distance between the camera and the subject may be obtained by using a binocular stereo vision method using two cameras or a triangulation method using structured light and a camera. However, in the binocular stereo vision method or the triangulation method, as the distance between the camera and the subject increases, accuracy of the information about the distance is rapidly reduced, and the distance measured is dependent on the surface state of the subject. Thus it is difficult to obtain accurate information about the distance.
In order to overcome this problem, a time-of-flight (TOF) method has been developed. In the TOF method, a light flight time, which is the time taken for modulated light to be irradiated onto a subject, reflected from the subject and received by a light receiving unit, is measured.
TOF methods may be classified into a TOF method using direct time measuring, a TOF method using phase delay measuring, and a TOF method using correlation.
In the TOF method using direct time measuring, the time taken for pulse light to be projected onto a subject and reflected from the subject is measured using a timer.
In the TOF method using phase delay measuring, a light having a continuous sinusoidal wave is projected onto a subject, reflected by the subject, and a phase difference of the light is detected to calculate distance.
In the TOF method using correlation, pulse light is projected onto a subject and the distance between the camera and the subject is measured by using information about brightness that is obtained by multiplying a gating signal that is synchronized with the pulse light by a reflection light signal. In the TOF method using correlation, the amount of light of a near subject that is received by the camera is large and thus the image of the subject looks bright, whereas the amount of light of a far subject that is received by the camera is small and thus the image of the subject looks relatively dark.
In the TOF method using phase delay measuring or the TOF method using correlation, information about the distance may be obtained using high-speed gating using an optical modulator.
An image amplifier, which is a type of optical modulator, may obtain a bright image by amplifying fine light by performing a three-step boosting operation. However, an operating voltage of the image amplifier has to be high, and the image amplifier is large sized. In the image amplifier, gating has to be performed so as to obtain information about the distances between a camera and a subject and may be performed using a photocathode of the image amplifier at a speed of about 1 ns to about 5 ns. However, even after gating off is performed using the photocathode of the image amplifier, charges remain in phosphor of the image amplifier. Thus, gating is not completely performed in the image amplifier until light due to the charges that remain in the phosphor of the image amplifier disappears. As such, gating speed of the image amplifier is about 30 Khz to several hundreds of Khz.
An optical modulator using a Pockel or Kerr effect based on crystal optics uses variation of refractive indices according to voltages. In the optical modulator using the Pockel or Kerr effect based on crystal optics, gating may be performed at a speed of several GHz, and a high control voltage of several KV is necessary.
Also, a semiconductor-based optical modulator using a multiple quantum well (MQW) is small-sized and may be driven at low voltage. However, in the semiconductor-based optical modulator using the MQW, an optical gain and wavelength conversion structure is not present, and an ON/OFF ratio is low, and it is difficult to maintain the semiconductor-based optical modulator using the MQW in a completely OFF state. Also, a pixelation operation and a dedicated design of electrodes for operation may be performed so as to gate a large-scale device at high speed.
A semiconductor-based optical shutter having an optical gain includes an Avalanche photodiode (APD) and a light emitting diode (LED), which are vertically stacked on a semiconductor substrate. The semiconductor-based optical shutter may have a small volume. However, a current loop is formed via a gate during a gating operation, and current generated in the APD is diverged by the LED and the gate so that a loss of current may occur. Also, the gate itself may be turned on/off at high speed, and the gating speed of the semiconductor-based optical shutter is reduced because it takes time to turn off the LED. In other words, charges are not transferred to the LED from the APD in a gating ON state, and it takes time for light due to charges stacked on the LED before the gating ON state to disappear. As a result, an optical tail is generated.